MilCohort Overview Feb07 J Clin Epidemiol

Journal of Clinical Epidemiology 60 (2007) 181e191

Millennium Cohort: enrollment begins a 21-year contribution to
understanding the impact of military service
Margaret A.K. Ryana,*, Tyler C. Smitha, Besa Smitha, Paul Amorosob, Edward J. Boykoc,
Gregory C. Grayd, Gary D. Gackstettere,y, James R. Riddlef, Timothy S. Wellsf, Gia Gumbsa,
Thomas E. Corbeila, Tomoko I. Hoopere
a

Department of Defense Center for Deployment Health Research, Naval Health Research Center, P.O. Box 85122, San Diego, CA 92186-5122, USA
b
Army Research Institute of Environmental Medicine, Military Performance Division, Natick, MA, USA
c
Seattle Epidemiologic Research and Information Center, Veterans Affairs Medical Center, Puget Sound, Seattle, WA, USA
d
Department of Epidemiology, College of Public Health, University of Iowa, Iowa City, IA, USA
e
Department of Preventive Medicine and Biometrics, Uniformed Services University of the Health Sciences, Bethesda, MD, USA
f
Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA
Accepted 7 May 2006

Abstract
Objective: In response to health concerns of military members about deployment and other service-related exposures, the Department
of Defense (DoD) initiated the largest prospective study ever undertaken in the U.S. military.
Study Design and Setting: The Millennium Cohort uses a phased enrollment strategy to eventually include more than 100,000 U.S.
service members who will be followed up through the year 2022, even after leaving military service. Subjects will be linked to DoD and
Veterans Affairs databases and surveyed every 3 years to obtain objective and self-reported data on exposures and health outcomes.
Results: The first enrollment phase was completed in July 2003 and resulted in 77,047 consenting participants, well representative of both
active-duty and Reserve/Guard forces. This report documents the baseline characteristics of these Cohort members, describes traditional,
postal, and Web-based enrollment methods; and describes the unique challenges of enrolling, retaining, and following such a large Cohort.
Conclusion: The Millennium Cohort was successfully launched and is becoming especially relevant, given current deployment and
exposure concerns. The Cohort is representative of the U.S. military and promises to provide new insight into the long-term effects of
military occupations on health for years to come. Ó 2007 Elsevier Inc. All rights reserved.
Keywords: Military medicine; Military personnel; Veterans; Longitudinal studies; Combat disorders; Gulf War syndrome

1. Introduction
Since the 1991 Gulf War, numerous studies and much
effort has been expended to evaluate the health concerns
of veterans. Several large epidemiologic studies have found
no unexplained increase in morbidity among these veterans
[1e6], and etiologies for increased symptom reporting remain elusive after more than a decade [7e12]. Some hypothesize that symptoms and symptom complexes among
Gulf War veterans result from a more physically and psychologically demanding lifestyle in the military compared
with the typical experiences of the civilian working

y
Present address: Analytic Services, Inc. (ANSER), 2900 S. Quincy
St, Suite 800, Arlington, VA 22206.
* Corresponding author. Tel.: 619-553-8097; fax: 619-553-7601.
E-mail address: [email protected] (M.A.K. Ryan).

0895-4356/07/$ e see front matter Ó 2007 Elsevier Inc. All rights reserved.
doi: 10.1016/j.jclinepi.2006.05.009

population [13,14]. Others suggest that the psychological
and physical effects of deployment may have a greater impact on health [15e25]. Separation from family during prolonged deployments, irregular working hours, strenuous
training, mastering technologically advanced weaponry,
threats of exposure to unknown chemical or biological
agents, witnessing extreme violence and death, and dynamics inherent to deployment missions may all contribute to
increased symptom reporting or psychological distress during and after deployment [17e25]. Observations that U.S.
Reserves/National Guard military personnel may be at
greater risk for postdeployment illnesses have been of
particular concern [1,6,8,12,26e28].
In response to the Department of Defense (DoD) recommendation for a coordinated effort to study, the potential
effects of deployment-related exposures [29], and bolstered
by the Institute of Medicine’s recommendation for

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M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

a systematic, longitudinal, population-based assessment of
service members’ health [30], the Millennium Cohort
Study was launched in October 2000 [31]. Just as other
Cohort studies have yielded important findings for the development of public health policy [32e35], the Millennium
Cohort Study is poised to do the same. This prospective
study incorporates temporal sequence for the study of potential causal pathways of deployment-related exposures
and subsequent health outcomes. Ultimately it may improve the health of future service members by identifying
both risk factors and preventive factors for chronic disease.
In this paper, the authors present the successes and challenges of enrolling more than 77,000 service members in
the first panel of the Millennium Cohort.

2. Materials and methods
2.1. Study population
The invited first panel of the Millennium Cohort included 256,400 randomly selected U.S. military personnel.
To ensure adequate power for statistical inferences, several
subgroups were oversampled, including those previously
deployed, Reserve/Guard personnel, and female service
members. The probability-based sample, representing approximately 11.3% of the 2.2 million men and women in
service as of October 1, 2000, was provided by the Defense
Manpower Data Center (DMDC) in California. DMDC data
included sex, birth date, highest education level, marital
status, race/ethnicity, recent deployment to southwest Asia,
Bosnia, or Kosovo, pay grade, service component (active
duty and Reserve/Guard), service branch (Army, Navy,
Coast Guard, Air Force, and Marines), primary and duty
occupations, unit identification code, date and reason for
separation from service, Social Security number (SSN),
name, and home and duty addresses.
2.2. Focus groups and pilot study
To improve the survey instrument, enhance tracking and
database processing techniques, establish practical timelines, evaluate potential cost-savings initiatives, and test
quality control measures for combining paper and electronic questionnaire submissions, three focus groups were
conducted and the survey was pilot tested on approximately
1% of the initial sample. The resulting group of nonresponders afforded the opportunity to investigate reasons
for nonresponse among 100 randomly chosen individuals
by phone interview. These findings were then used to
further refine the instrument and enrollments methods.
2.3. Enrollment invitations
Initial enrollment of the Millennium Cohort began with
postcard mailings in July 2001, followed shortly by the terrorist attacks of September 11 and the anthrax attacks

through the U.S. Postal Service (USPS). To address these
challenges, the research team used a modified Dillman
method with extended enrollment cycles [36]. Both the initial and extended enrollment cycles included an introductory postcard, survey, and reminder postcard mailings
outlined by Dillman, with repeat survey and reminder postcard mailings for nonresponders. To ensure adequate time
for locating new addresses and processing returned mail,
enrollment cycles were staggered, with each cycle lasting
approximately 7 months. The final invitation to join the
Cohort was mailed in December 2002.
Recognizing the possible limitations of enrollment
through mail surveys alone due to increased deployment
of service members, the team added an e-mail invitation
cycle. E-mail invitations encouraged participation via the
World Wide Web (Web) by providing a direct link to the
online survey, but also encouraged completion of the paper
surveys for those preferring that option. To further compensate the increase in military deployments and the highly
mobile nature of our target population, the team added an
extended enrollment cycle that mirrored the procedures
used in the initial enrollment cycle.
2.4. Web enrollment
The growing ubiquity of e-mail and the web provided an
alternate contact and recruitment modality. Although U.S.
mail service to bases and camps overseas may be slow
and sporadic, many deployed U.S. military personnel have
Web and email access. The demonstrated shortcomings
of standard mail, the clear benefit to data integrity, and
substantial cost savings made Web enrollment especially
advantageous. Due to the sensitive nature of these data, secure data transmission of survey responses via the Web was
of paramount importance. Web site security licenses were
sought and each participant’s user identification and password were verified prior to granting access to the survey
Web site. All transmissions between the participant’s Web
browser and the Web server software were based on the
most secure technology available at the time, using well
established and widely accepted Secure Sockets Layer
technology with 128-bit encryption.
2.5. Cost-savings initiatives
The Web-based survey option was encouraged by the research team because of its potential to increase data quality
and reduce costs. The Web site address was highlighted on
mailed correspondence, and e-mail communication provided direct links to the survey log-in page. In addition,
a free T-shirt or phone card was offered to those choosing
the Web option. These cost-saving initiatives, or incentives,
proved effective in increasing Web-based response steadily
throughout the enrollment period, resulting in more than
half of all participants completing their questionnaires
online.

M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

Most project costs were considered to have a shared association between Web-based and paper-based respondents.
The study team conservatively estimated, however, that the
costs associated with paper-respondents alone included
questionnaire printing, return postage, scanning hardware
and software, paper storage facilities, and personnel time
for scanning, verifying, and filing the paper questionnaires.
Costs that were considered associated with web respondents alone included web questionnaire design and coding,
web security, server costs, and e-mail invitation costs. A
conservative estimate of the differential cost indicated that
Web response saved the project at least $50 over paperbased response.
2.6. E-mail and postal addresses
Algorithms were developed for efficient mail tracking
and for identifying accurate postal addresses. Initial addresses were obtained from DMDC, followed by inexpensive postcard mailings with return receipt to validate
addresses. Address locator services were used for invalid
addresses, including commercial locators and the Internal
Revenue Service (IRS) through an agreement with the National Institute for Occupational Safety and Health. Assessment of the address-finding services was conducted by
randomizing more than 375 service members with invalid
addresses into two groups, one forwarded to a commercial
locator and the other to the IRS. The IRS addresses were
found to be correct more often than those from the commercial locator (62% vs. 28%) and at a substantial cost savings.
E-mail addresses held the added advantage in providing
opportunities for increased contact with potential Cohort
members as well as the substantial cost savings associated
with the Web-based survey submission. Although ascertainment of e-mail addresses was challenging because DoDwide electronic databases maintained by DMDC were new
and evolving, e-mail addresses linked by SSN were obtained
for Cohort members from the Army, Air Force, Navy, and
Marines. In the future, because all military identification
cards are converted to the microchip-containing Common
Access Cards, DMDC will maintain a listing of current
e-mail addresses, linked by SSN, for all service members.
2.7. Participant tracking after enrollment
Semiannual e-mails and postcards are used to track participants, sustain interest in continued study participation,
and verify accuracy of contact information for this highly
mobile population. The research team selected Memorial
Day and Veterans Day to send postcards because these holidays may hold special significance for service members
and are spaced approximately 6 months apart. Each holiday
contact consists of a unique postcard and e-mail message
thanking subjects for their contribution to military service
and to the study, and directing them to the study Web site
to update their contact information. In addition to this

183

cost-effective means of verifying the accuracy of contact
information, the USPS ‘‘Return Service Requested’’ is used
to obtain forwarding address information on undeliverable
postcards.
2.8. Survey instrument
The Millennium Cohort questionnaire for first enrollment included more than 450 questions on diagnosed medical conditions, symptoms, psychosocial assessment,
physical status, functional status, use of alcohol, tobacco,
complementary and alternative medicine, occupations, military exposures, sleep patterns, and basic demographic and
contact information [31]. Standardized instruments were incorporated whenever possible because of their established
reliability and validity and to enable future comparisons
with other populations. Such instruments included the Primary Care Evaluation of Mental Disorders (PRIME-MD)
Patient Health Questionnaire (PHQ) [37e39], used to assess major depressive syndrome, panic syndrome, other
anxiety syndrome, bulimia nervosa, alcohol abuse, and
binge-eating disorders, (overall accuracy 5 0.85, 85%);
(sensitivity 5 0.75, specificity 5 0.90), as well as specific
conditions such as major depressive disorder (sensitivity 5 0.93, specificity 5 0.89) [40], and panic disorder
(sensitivity 5 1.00, specificity 5 0.63) [41]; the Medical
Outcomes Study Short Form-36 for Veterans (SF-36V)
[42,43] (eight components to assess physical functioning,
role limitations caused by physical problems, bodily pain,
general health, vitality, social functioning, role limitations
caused by emotional problems, and mental health) found
to have high internal consistency across all eight domains
in a military population [44]; a Department of Veterans Affairs Gulf War survey of specific war-time exposures such
as depleted uranium, and chemical or biological warfare
agents [8,45]; the CAGE questionnaire for the detection
of alcoholic problems [46]; and the Posttraumatic Stress
Disorder (PTSD) ChecklisteCivilian Version (PCL-C)
[47e49] shown to be highly specific (specificity 5 0.99)
with slightly lower sensitivity (60%), a positive predictive
value of 75%, and negative predictive value of 97% when
using a cutoff of 50 [50]. Participants were identified as
possibly having PTSD if they reported experiencing (at
moderate or more extreme level) at least one intrusion
symptom, three avoidance symptoms, and two hyperarousal
symptoms [51], and had a total score of 50 or more on
a scale of 17e85 [16,48,52,53]. Free text fields were included to allow participants to report conditions, problems,
concerns, and exposures not listed elsewhere on the survey.
2.9. Data quality monitoring
To ensure the highest quality data, systematic validation
and quality control processes were established for both
paper and electronic submissions. The paper survey was
created, scanned, and verified using mark-sense TELEform

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M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

Elite Software (Cardiff Software, Vista, CA, USA). For
every 3,000-paper surveys that were scanned, 50 were
randomly selected and compared with corresponding electronic records, with necessary adjustments made. In addition, all electronic records from scanned paper survey
responses were searched for excessive missing values and
then verified that the responses were truly missing. If, in
fact, the responses were present (typically, very light pencil
response was the cause), data were entered manually. The
sensitivity of the scanning device was assessed throughout
the process of scanning paper surveys, and thresholds were
set for missing data that triggered further investigation.
The electronic version of the survey facilitated quality
control by allowing direct data entry by responders. To ensure that all fields properly translated electronically, numerous mockup surveys were submitted both initially and
throughout enrollment to check for correct data coding
and transmission. Trends in missing data over the length
of the survey that might indicate diminishing interest or
survey fatigue were not detected (Fig. 1). Although no
question was skipped by more than 15% of the responders,
several questions tended to be associated with missing
responses proportionally more often than other questions.

This prompted review of these questions for follow-up
surveys.
2.10. The Cohort
Although the initial target population included 256,400
potential participants, 1,270 were excluded due to invalid
SSNs, name, or address information; 2,560 were included
in the pilot study [31]; and 38,182 could not be contacted
after multiple address searches found no valid address
(Fig. 1).
Among the 214,338 contacted members of the invited
sample, enrollment in the Cohort was not completed for
the following reasons: determined to be ineligible (n 5 8),
deceased (348), explicitly declined to participate
(n 5 4,796), survey completed by someone other than the
invited service member (n 5 83), consent form not returned
(n 5 2,208), and survey submitted after close of enrollment
(n 5 11). All others were considered to be nonresponders
(n 5 129,887). The 77,047 consenting participants in the
first enrollment panel represent 36% response rate of those
contacted and invited to enroll.
2.11. Analyses

Original sample
N = 256,400
Incomplete addresses, duplicate records
(n = 1,270)
Pilot study (n = 2,560)
Last address returned undeliverable
(n = 38,182)
Panel 1 subjects contacted
N = 214,388
Ineligible (n = 91)

Descriptive analyses of the initial Cohort included
means and proportions of important demographic characteristics as well as selected survey questions of interest. Initial results were stratified by active-duty or Reserve/Guard
status. Univariate statistics including chi-square and t tests
of association were used to establish differences among the
enrolled Cohort members, the invited sample, and the U.S.
military in 2000. Data were warehoused and analyzed using
SAS software (Version 9.1, SAS Institute, Inc., Cary, NC,
USA) [54,55].

Deceased (n = 348)
Declined to participate/ Refused
(n = 4,796)
Nonresponders (n = 129,887)
Responses
N = 79,266
Nonconsented responses (n = 2,208)
Consented responses
N = 77,058

Survey received after July 1, 2003
(n = 11)

PANEL 1
N = 77,047

Fig. 1. Millennium Cohort Study flow of participants from sample to
enrolled Cohort with Millennium Cohort Study logo.

3. Results
Demographic data for the Cohort were complete for
76,715 of the 77,047 (99.6%) participants (Table 1). Univariate analyses of population demographics suggested that
there were statistically significant differences between the
Cohort, invited sample, and the composition of the 2000
U.S. military (Table 1). When compared with the 2000
U.S. military at large, Cohort members were slightly more
likely to be female, older, better educated, married, officers,
in the Air Force, and from health care occupations. The
higher enrollment of women and those recently deployed
reflects the intended oversampling.
Self-reported health behaviors and military exposures
are shown in Table 2. The most frequently reported military-specific exposures included receiving at least one anthrax vaccination (32%) and witnessing a person’s death
due to war, disaster, or tragic event (26%) (Table 2). Five
percent of the Cohort reported being exposed to chemical
or biological warfare agents and 4% reported being

Table 1
Characteristics of Millennium Cohort Study responders (panel 1), compared to the invited sample and the U.S. military, as of October 2000
Variable

Cohort, N 5 77,047; N (%)

Invited Cohort,a N 5 256,400; (%)

U.S. military,b N 5 2,273,793; (%)

Sex
Male
Female
Unknown

56,415 (73.2)
20,632 (26.8)
0 (0.0)

76.0
24.0
!0.1

84.7
15.3
!0.1

Age, years
17e24
25e34
35e44
O44
Unknown

14,559
27,083
25,400
9,975
30

(18.9)
(35.2)
(33.0)
(13.0)
(!0.1)

30.8
35.4
25.1
8.6
0.1

32.5
33.9
25.0
8.5
0.1

Education
Less than high school diploma
High school diploma
Some college
Bachelor’s degree
Master’s/PhD degree
Unknown

4,722
32,957
19,655
12,722
6,986
5

(6.1)
(42.8)
(25.5)
(16.5)
(9.1)
(!0.1)

7.6
50.4
23.6
11.6
5.4
1.4

8.0
53.0
20.3
11.3
5.8
1.6

Marital status
Single
Married
Divorced
Unknown

23,183
48,594
5,270
0

(30.1)
(63.1)
(6.8)
(0.0)

40.5
52.8
5.7
1.0

41.7
53.2
5.0
0.2

Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Native American
Other
Unknown

53,459
10,576
6,068
4,921
677
1,065
281

(69.4))
(13.7)
(7.9)
(6.4)
(0.9)
(1.4)
(0.4)

64.7
19.0
6.1
7.5
0.9
1.5
0.4

67.3
18.7
3.3
7.9
1.0
1.2
0.8

Past deployment status
Deployment experience
No deployment experience

23,234 (30.2)
53,813 (69.8)

30.0
70.0

10.0
90.0

Military pay grade
Enlisted
Commissioned officer
Warrant officer

59,318 (77.0)
16,346 (21.2)
1,383 (1.8)

84.6
14.3
1.1

84.3
14.5
1.2

Service component
Active duty
Reserve/Guard

43,890 (57.0)
33,157 (43.0)

54.9
45.1

57.5
42.5

Branch of service
Army
Air Force
Navy
Marines
Coast Guard
Unknown

36,481
22,357
13,435
3,941
833
0

(47.4)
(29.0)
(17.4)
(5.1)
(1.1)
(0.0)

44.0
28.1
19.6
7.2
1.2
1.2

45.5
23.3
20.0
9.4
1.9
!0.1

Occupational category
Combat specialists
Electrical repair
Communications/intelligence
Health care specialists
Other technical
Functional support specialists
Electrical/mechanic
Craft workers
Service support
Students, prisoners, other
Unknown

15,425
6,784
5,428
8,018
1,972
15,413
11,387
2,390
6,686
3,523
21

(20.0)
(8.8)
(7.1)
(10.4)
(2.6)
(20.0)
(14.8)
(3.1)
(8.7)
(4.6)
(!0.1)

20.9
8.0
6.7
8.4
2.4
17.9
16.2
3.5
8.9
5.8
1.3

21.9
8.1
7.0
8.1
2.7
17.6
15.1
3.7
9.4
3.7
2.8

a
b

Oversampled for women, recently deployed, and Reserve/Guard.
Based on 2000 U.S. military service rosters.

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M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

Table 2
Examples of unadjusted survey datadexposures and health behaviors among Millennium Cohort members upon enrollment
Cohort, N 5 77,047; N (%)
Self-reported military exposures
Ever exposed to the following:
Witnessed a person’s death due to war,
disaster, or tragic event
Chemical or biological warfare agents
Anthrax vaccine
Exposed within the past 3 years:
Depleted uranium
Occupational hazards requiring protective
equipment, such as respirators or
hearing protection
Any exposure, physical or psychological,
during a military deployment that had
a significant impact on your health
Behavioral risk factors
Alcohol
Chronic drinkersa
Drank five or more drinks on >1 day(s)
in past year

Active duty, N 5 43,890; (%)

Reserve/Guard, N 5 33,157; (%)

19,621 (25.5)

(25.1)

(26.0)

4,175 (5.4)
24,701 (32.1)

(4.8)
(43.1)

(6.3)
(17.4)

2,826 (3.7)
41,430 (53.8)

(4.8)
(59.2)

(2.2)
(46.7)

5,181 (6.7)

(7.3)

(5.9)

5,801 (7.5)
35,195 (45.7)

(7.5)
(48.2)

(7.6)
(42.3)

Smoking
Never smoked O100 cigarettes in lifetime
Smoked O100 cigarettes in lifetime
Unknown
Pack-years (median, IQR)b

42,557
31,460
3,030
5.3

(55.2)
(40.8)
(3.9)
(12.3)

(55.5)
(41.0)
(3.5)
4.5 (10.0)

(54.9)
(40.6)
(4.5)
6.0 (13.5)

BMI (kg/m2)
Underweight (!18.5)
Normal (18.5e24.9)
Overweight (25.0e29.9)
Obese (>30.0)
Unknown

636
27,758
39,194
8,394
1,065

(0.8)
(36.0)
(50.9)
(10.9)
(1.4)

(0.8)
(36.6)
(51.3)
(10.0)
(1.3)

(0.9)
(35.3)
(50.3)
(12.0)
(1.5)

(8.0)
(8.6)
(1.7)

(14.8)
(10.7)
(1.7)

Complementary and alternative medicine use
Chiropractic care
Herbal therapy
Acupuncture

8,424 (10.9)
7,312 (9.5)
1,289 (1.7)

Abbreviation: IQR, interquartile range.
a
Chronic drinkers defined as O14 drinks in a typical week for men, and O7 drinks in a typical week for women.
b
Median pack-years and IQR calculated for participants smoking O100 cigarettes in their lifetime.

exposed to depleted uranium. Nearly three times the proportion of active-duty personnel had reported receiving
the anthrax vaccine (43%) compared with Reserve/Guard
personnel (17%).
Regarding modifiable risk behaviors, 46% of participants might be categorized as binge drinkers, whereas only
8% were categorized as chronic drinkers (Table 2). Activeduty and Reserve/Guard subjects appeared similar with regard to chronic use of alcohol; however, more active-duty
members were classified as binge drinkers in the past year
(48% and 42%, respectively). Approximately 40% of participants smoked at least 100 cigarettes in their lifetime
(median pack-years 5 5.3), whereas the majority of the Cohort (55%) had not smoked. Although active-duty and Reserve/Guard responders were similar with respect to having
ever smoked 100 cigarettes in their lifetime, median packyears were higher in Reserve/Guard than active-duty

members (6.0% and 4.5%, respectively). Reserve/Guard
members were more likely to be obese as indicated by body
mass index (BMI) and also more likely to have used complementary and alternative medicine therapies.
More than 60% of the Cohort reported that their general
health was very good or excellent. There were 38 specific
medical conditions and one free text option included on
the questionnaire of which we report on one prevalent condition (hypertension, 10.2%), a condition of current high
public health concern (diabetes mellitus, 1.3%), and a condition of much interest to veterans of past deployments
(chronic fatigue syndrome, 1.3%) (Table 3). Major depressive disorder, as defined by the PRIME-MD PHQ, was
reported by 3.3% of the Cohort. The PCL-C responses suggested that 2% of the Cohort has signs or symptoms of
PTSD. The SF-36V assessment of functional status indicated relatively high means (range: 62.1e91.0), with the

M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

187

Table 3
Examples of unadjusted survey datadphysical and mental health among Millennium Cohort members upon enrollment
Cohort, N 5 77,047; N (%)
General health
In general, would you say your health is.
Poor
562 (0.8)
Fair
5,331 (7.1)
Good
23,373 (31.3)
Very good
30,437 (40.7)
Excellent
15,045 (20.1)

Active duty, N 5 43,890; %

Reserve/Guard, N 5 33,157; %

1.0
8.1
33.1
39.3
18.6

0.5
5.9
28.9
42.6
22.1

Specific medical conditions
Hypertension
Chronic fatigue syndrome
Diabetes

7,799 (10.2)
1,015 (1.3)
1,008 (1.3)

9.9
1.3
1.2

10.6
1.4
1.5

PRIME-MD PHQ
Major depressive disorder
Panic syndrome
Other anxiety syndrome
Eating disorders

1,002
890
1,617
2,457

(3.3)
(1.2)
(2.1)
(3.2)

3.6
1.2
2.4
3.5

2.9
1.2
1.8
2.9

PCL-C
PTSDa

1,821 (2.4)

2.5

2.3

Mean
90.6
81.6
73.8
75.5
60.7
86.4
83.7
78.1

Mean
91.6
83.1
77.5
78.7
64.1
88.2
83.7
79.4

b

SF-36V
Physical functioning
Role physical
Bodily pain
General health
Vitality
Social functioning
Role emotional
Mental health

Mean
91.0
82.2
75.4
76.9
62.1
87.1
83.7
78.6

Abbreviation: PCL-C, Posttraumatic Stress Disorder Checklist-Civilian Version.
a
PTSD, posttraumatic stress disorder defined as moderate or above level of at least one intrusion symptom, three avoidance symptoms, and two hyperarousal symptoms, with a total score >50 on a scale of 17e85.
b
SF-36V, Medical Outcomes Study Short Form-36 for Veterans. Increasing score indicates better health and functioning status, with a maximum score
of 100.

highest mean for physical functioning. SF-36V scores were
somewhat higher, indicating better functional status, for
Reserve/Guard than for active-duty responders (Fig. 2).

4. Discussion
The Millennium Cohort represents a major milestone in
military and occupational epidemiologic research. The prospective study design responds to an important charge to assess objective health information by linking to DoD
maintained inpatient, ambulatory, and pharmacy database,
as well as subjective symptoms and level of functioning,
among a large military population over several decades, during and beyond actual military service [30]. Health status is
assessed through triennial questionnaires as well as by linking to large health care databases. Likewise, exposure experiences are evaluated through triennial questionnaires as
well as linking to electronically maintained occupation, vaccine, deployment, and environmental exposure history data.
With those data that currently overlap while in military service, preliminary investigation of concordance between

self-report and electronic occupation and vaccine data suggest substantial reliability in these data. A vital component
of the study is the ability to compare Reserve/Guard forces
with regular active-duty forces on both exposures and health
outcomes. The Reserve/Guard represent the ‘‘citizen soldiers’’ and, as such, are an important comparison population
for those choosing the military as a full-time occupation.
Despite extensive planning and pilot testing, the Millennium Cohort enrollment year was marked by numerous
challenges. The study was launched shortly before the historic terrorist attacks of September 11 and the crippling effects of the anthrax scare on the U.S. postal system.
Investigators mitigated these challenges by extending the
invitation cycle, locating new addresses using the IRS, using e-mail as a contact mode, and encouraging enrollment
via a secure Web site. These strategies, as well as use of
semiannual Veterans Day and Memorial Day contact, will
be leveraged to maintain participation and complete follow-up of the original Cohort every 3 years through 2022.
These strategies will also be important in enrolling subsequent panels of the Cohort, in 2004 and one planned for
2007, to achieve a total enrollment of at least 140,000.

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M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191
100

Percent Complete

95

D
90
C
A
E

B

85

80

75
1

9

17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 185 193 201 209 217 225 233 241 249

Question number
Fig. 2. Individual question completion percentages of Millennium Cohort questionnaire. Percentages incorporate skip patterns. A: 13% skipped ‘‘other’’ on
the question ‘‘Has your doctor or other health professional ever told you that you have any of the following conditions?’’; B: 15% skipped ‘‘other’’ on the
question ‘‘During the last 12 months, have you had persistent or recurring problems with any of the following conditions?’’; C: 11% of those indicating eating
disorder skipped a frequency query on the problem; D: 9% of those who indicated problems skipped a query qualifying degree of challenge in ‘‘doing work,
taking care of things at home, or getting along with other people?’’; and E: 14% skipped a military occupational coding query.

Members of the Cohort were demographically older,
more educated, married, and in the officer ranks, compared
with individuals in the invited sample or the military population at large. The high operational tempo following the
September 11 terrorist attacks in 2001 may be one explanation for underrepresentation of Marines and those aged
17e24 years, as large numbers of young service members
participated in lengthy combat deployments; however, this
trend of lower participation among younger invitees has
been reported elsewhere [56]. Deployment aside, older
and more educated individuals and those classified as
health care specialists, may have more interest in health issues, perhaps increasing their propensity for enrollment.
The sampling strategy to ensure adequate representation
to assess rare outcomes in particular subgroups was largely
successful, with women comprising nearly 27% of the Cohort, whereas those with prior deployment experience comprise 30% of the Cohort. The small proportional differences
suggest that the Cohort is a reasonably representative sample of the military as a whole, and study findings should be
generalizable to the target population. Subsequent panel enrollment will allow investigators to reflect the changing
composition of the U.S. military.
Unlike the civilian workplace, there are inherent,
unique, and sometimes unpredictable, hazards associated
with military service [2,4,5,57]. More than half of the Cohort reported having used protective equipment because of
potential occupational hazards (Table 2). Also, witnessing
a tragic event, including death, can be an accompaniment
of military service. About 25% of both the active-duty
and Reserve/Guard components of the Cohort have personally experienced such events. Other potentially hazardous

exposures associated with military service include the target
of lethal weapons, operating sophisticated weapons systems,
and working under environmentally extreme conditions
[58]. Finally, deployed military personnel are exposed to
specific pharmaceuticals, multiple immunizations, and other
products, which are rarely, if ever, administered to civilians
[58e60]. The Millennium Cohort allows, for the first time,
the opportunity to assess such exposures prospectively on
a large sample.
The Millennium Cohort has the advantage of being systematically drawn from all branches and components of the
armed forces, using repeated measures to monitor population trends over at least two decades. Individual selfreported behavioral data may be linked to specific and
militarily relevant health outcomes, even among those
who retire or otherwise leave military service. Robust comparisons between Reserve/Guard and active forces have
heretofore not been possible because a standardized instrument has never been applied to study their similarities and
differences in such a systematic and comprehensive fashion. Although explicit comparisons, adjusted for factors
such as age and sex, are beyond the scope of this introductory paper, the preliminary data presented here suggest that
military personnel will report health and behavioral habits
even when these habits are relatively unhealthy. Early data
suggest that there will be large subgroups of military personnel in different risk categories that may influence the
occurrences of illnesses and injuries. For example, data
from this baseline survey show that more than 50% of
the Reserve/Guard and active component members of the
Millennium Cohort are overweight but that only 10% are
considered obese, with a slightly higher proportion of the

M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

Reserve/Guard exceeding the BMI for obesity (Table 2).
When compared with the Healthy People 2010 objectives,
Millennium Cohort participants meet or surpass the objectives for alcohol moderation and weight control [61].
Given the relative youth of the Cohort and the physical
fitness standards that must be met for military service, one
might expect the general health among military service
members to be considerably better than that of the U.S. average. In fact, over 90% of the Cohort rated their physical
health as good or better, compared with 85% of U.S. residents included in the 2003 Behavioral Risk Factor Surveillance System (BRFSS) [61,62]. This was true for both
Reserve/Guard and active-duty personnel. Of note, 51%
of 2003 BRFSS participants were younger than 45 years
of age compared to 87% of Cohort members. Similar high
ratings of self-reported health have been reported for active-duty military personnel who participated in the 2000
BRFSS [63] as well as military personnel who returned
from international deployments and were part of the Defense Medical Surveillance System, the central repository
of U.S. military medical surveillance data [64]. However,
active-duty military personnel were more likely to report
greater number of days of activity limitation, pain, and
not enough rest than their counterparts who were not in
the military [63]. This appears to be consistent with relatively lower SF-36V scores for physical functioning, bodily
pain, and general health in active duty when compared with
Reserve/Guard responders (Table 3). Finally, the low rates
of chronic conditions such as diabetes and hypertension
were as expected, given the relative youth of the Cohort
[65]. Self-reported prevalence of nonpregnancy-related diabetes and hypertension in the 2003 BRFSS (7% and 25%,
respectively) was considerably higher than that seen in this
Cohort [62]. A slightly higher prevalence of hypertension
and diabetes was seen in Reserve/Guard vs. active-duty
members, possibly explained by the somewhat higher prevalence of obesity (BMI > 30) in Reserve/Guard (Table 2).
In addition to describing baseline mental and physical
health, this is the first population-based mental health survey of all components of the U.S. military (active duty,
Guard and Reserve) that documents the substantial burden
of symptomatic mental illness among all U.S. military
members at rates similar to that of the general U.S. population. Worldwide, mental disorders accounted for nearly
11% of the disease burden in 1990 and are projected to affect 15% of the world population by 2020, causing a public
health impact nearly as large as cardiovascular and respiratory diseases [66,67]. Thirteen percent of all military hospitalizations from 1990 to 1999 were reported as mental
health disorders [68], and as many as 17% of serving members had symptoms of anxiety and 19% had symptoms of
depression in 2001 [69]. Among Cohort members, meaningful levels of a number of common, potentially serious
mental disorders were identified and found to be consistent
with prevalence in other populations, such as major depressive disorder (3%), panic syndrome (1%), other anxiety

189

syndrome (2%), eating disorders (2%), and PTSD (2%).
Future analyses will provide insight into risk factors that
may be used to target groups at highest risk for intervention, as well as to discern the impact of deployment on
mental health. Finally, the Cohort exhibited higher unadjusted means measured for SF-36V physical functioning,
general health, vitality, social functioning, role emotional,
and mental health, and lower unadjusted means for those
components describing role limitations due to physical
problems and bodily pain, suggesting a more functionally
capable population when compared with the U.S. population [42].
The Millennium Cohort Study represents the first ever,
comprehensive effort by any nation to prospectively evaluate health outcomes of military service. This project holds
tremendous promise to help us better understand enigmatic
problems, such as multisymptom illnesses experienced by
Gulf War veterans. The Cohort will also identify and characterize the numerous, some as yet unidentified, benefits to
health that may be common to our men and women in uniform, but not detected by previous study methodologies.
The enrollment of more than 60,000 additional service
members in subsequent panels will ensure that the Cohort
remains relevant and representative of the military and their
experiences with current and future deployments. Like
other groundbreaking prospective studies, the value of the
Millennium Cohort in defining causes of both health and
disease is expected to have a resounding impact that grows
over time.

Acknowledgments
This represents report 05-17, supported by the DoD, under work unit no. 60002. The views expressed in this article
are those of the authors and do not reflect the official policy
or position of the Department of the Navy, Department of
the Army, Department of the Air Force, Department of
Defense, Department of Veterans Affairs, or the U.S. Government. Approved for public release; distribution is
unlimited. This research has been conducted in compliance
with all applicable federal regulations governing the
protection of human subjects in research (Protocol
NHRC.2000.007).
We thank Scott L. Seggerman from the Management
Information Division, Defense Manpower Data Center,
Seaside, CA; Karen Chesbrough, Laura Chu, Isabel Jacobson, Sheila Jackson, Cynthia Leard, Travis Leleu, Nick
Martin, Robb Reed, Tony Russo, Steven Speigle, Jim
Whitmer, Christina Spooner, and Dr. Sylvia Young, Department of Defense Center for Deployment Health Research at
the Naval Health Research Center, San Diego, CA; Dr.
Nicole Bell and Laura Senier, Army Research Institute of
Environmental Medicine, Total Army Injury and Health
Outcomes Database Project, Natick, MA; and Dr. Charles
C. Maynard, Department of Health Services, University

190

M.A.K. Ryan et al. / Journal of Clinical Epidemiology 60 (2007) 181e191

of Washington School of Public Health and Community
Medicine, Seattle WA. We appreciate the support of the
Henry M. Jackson Foundation for the Advancement of Military Medicine, Rockville, MD. We are extremely grateful
to the current and past members of the Millennium Cohort
Scientific Steering and Advisory Committee: Dr. Elizabeth
Barrett-Connor, University of California San Diego; Dr.
Dan G. Blazer, Duke University Medical Center; Dr.
Laurence G. Branch, College of Public Health, University
of South Florida; Dr. Bradley N. Doebbeling, Indiana University School of Medicine; Dr. Harold M. Koenig, Edward
Martin & Associates; Shannon Middleton, the American
Legion; Michael J. O’Rourke, Veterans of Foreign Wars;
Dr. Lawrence A. Palinkas, University of California San
Diego; Al Pavich, Vietnam Veterans of San Diego; Dr.
Michael Peddecord, San Diego State University; Dr. John
D. Potter, Fred Hutchinson Cancer Research Center; Joseph
C. Sharpe, Jr., the American Legion; Lisa Spahr, the American Legion; Dr. G. Marie Swanson, University of Arizona
College of Public Health; and Dr. Noel Weiss, University of
Washington School of Public Health.

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